The invention relates to a velocity change sensor of the kind especially adapted for use with an automotive vehicle equipped with a passenger restraint, such as an inflatable air bag, the sensor being operable in response to a change in velocity of the vehicle of predetermined magnitude and duration to initiate operation of the restraint and provide protection for an occupant of the vehicle.
A sensor constructed in accordance with the invention has a movable acceleration sensing mass that is magnetically biased to and maintained in an inactive position until such time as it is subjected to acceleration in excess of the magnetic biasing force, whereupon the sensing mass may move from its inactive position toward a second position in which it is operable to initiate operation of a restraining device. The movement of the sensing mass toward its operative position is damped, thereby ensuring that the restraint device will not be actuated unless the change in velocity of the vehicle is of sufficient magnitude and occurs over a sufficiently short period of time to require operation of the restraint to protect an occupant of the vehicle.
It is generally accepted that an occupant of an automotive vehicle is likely to be injured if the vehicle is involved in a crash and is decelerated sufficiently rapidly to cause the occupant to impact a structural part of the vehicle, such as the dashboard or windshield, at about twelve miles per hour or more. If the occupant is to be protected under these conditions, it is imperative that the velocity change of the vehicle be sensed in such manner as to predict the existence of circumstances which will lead to occupant injury and initiate deployment of an occupant protective device in sufficient time to prevent the occupant's striking a structural part of the vehicle at twelve or more miles per hour. On the other hand, a vehicle may be subjected to a deceleration pulse of considerable magnitude, but the duration of such a pulse may be insufficient to cause a twelve miles per hour velocity change between the vehicle and the occupant. In these circumstances, deployment of the restraining device is unnecessary. Thus, an acceptable crash sensor is one which is capable of distinguishing between acceleration pulses in which occupant protection is and is not required.
Crash sensors heretofore proposed for use in actuating vehicle occupant restraint systems are of three kinds. One is an electronic sensor which has certain cost objections. The second is a sensor based on inertial flow of a liquid, such as that described in U.S. Pat. No. 3,889,130. The third is a sensor having an acceleration sensing mass on which a biasing force is imposed by a spring. Examples of spring biased sensors appear in U.S. Pat. Nos. 3,380,046; 3,889,130; 3,974,350; and 4,097,699; and copending application Ser. No. 37,524, filed May 9, 1978 now U.S. Pat. No. 4,284,863.
Spring biased sensors have achieved the greatest acceptance, but the utilization of a spring for the initial biasing force does have certain characteristics which must be overcome. For example, the force required to compress a compression spring increases as the spring is compressed. Thus, the biasing force exerted on the sensing mass by a relatively uncompressed spring is less than that exerted when the spring is compressed. As a consequence, the biasing force exerted by a spring on an acceleration sensing mass varies in response to movement of the mass and, in particular, increases to a maximum during the acceleration pulse, rather than being at a maximum at the beginning of the pulse, as is preferable.
During acceleration due to certain kinds of crashes, it is possible that the vehicle may be braked so that the crash acceleration, coupled with that due to braking, is sufficient to generate the twelve miles per hour relative velocity between the vehicle and an occupant. Braking alone of a vehicle, however, would not require deployment of the occupant restraint. Thus, a velocity change sensor used to activate a restraint device should be one which is so constructed that it will not commence operation until the acceleration to which it is subjected is somewhat above the maximum obtainable from braking.
A common value used for the co-efficient of braking friction is 0.7. Thus, the maximum acceleration due to braking may be considered to be 0.7 G, where G means acceleration due to gravity. Occasionally, somewhat higher values have been measured, but it generally is assumed that braking acceleration will never exceed 1 G. On the other hand, it can be shown that, at a constant acceleration of 2.4 Gs, a front passenger seat occupant of a typical larger vehicle will strike a structural part of the vehicle at a relative speed of twelve miles per hour after traveling twenty-four inches. It is desirable, therefore, that the initial bias on the crash sensor for such vehicles be no more than about 2.4 Gs. Constant acceleration pulses rarely occur in actual crash conditions. Nevertheless, it has been found that very reliable results can be achieved by imposing an initial biasing force on the sensor of less than 3 Gs, and preferably of about 2 Gs, and reducing the final biasing force on the sensor to about 1 G, thereby always maintaining a biasing force on the sensor greater than the acceleration due to braking.
In some of the smaller vehicles the distance between a front seat passenger and the vehicle's dashboard or a windshield is less than twenty-four inches. Sensors adapted for use in such vehicles will utilize a higher initial biasing force, but it does not appear that a biasing force of 5 Gs need be exceeded.